Python 3.6.5 Documentation >  "copy" — Shallow and deep copy operations

"copy" — Shallow and deep copy operations
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**Source code:** Lib/copy.py

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Assignment statements in Python do not copy objects, they create
bindings between a target and an object. For collections that are
mutable or contain mutable items, a copy is sometimes needed so one
can change one copy without changing the other. This module provides
generic shallow and deep copy operations (explained below).

Interface summary:

copy.copy(x)

Return a shallow copy of *x*.

copy.deepcopy(x)

Return a deep copy of *x*.

exception copy.error

Raised for module specific errors.

The difference between shallow and deep copying is only relevant for
compound objects (objects that contain other objects, like lists or
class instances):

* A *shallow copy* constructs a new compound object and then (to the
extent possible) inserts *references* into it to the objects found
in the original.

* A *deep copy* constructs a new compound object and then,
recursively, inserts *copies* into it of the objects found in the
original.

Two problems often exist with deep copy operations that don’t exist
with shallow copy operations:

* Recursive objects (compound objects that, directly or indirectly,
contain a reference to themselves) may cause a recursive loop.

* Because deep copy copies everything it may copy too much, such as
data which is intended to be shared between copies.

The "deepcopy()" function avoids these problems by:

* keeping a “memo” dictionary of objects already copied during the
current copying pass; and

* letting user-defined classes override the copying operation or the
set of components copied.

This module does not copy types like module, method, stack trace,
stack frame, file, socket, window, array, or any similar types. It
does “copy” functions and classes (shallow and deeply), by returning
the original object unchanged; this is compatible with the way these
are treated by the "pickle" module.

Shallow copies of dictionaries can be made using "dict.copy()", and of
lists by assigning a slice of the entire list, for example,
"copied_list = original_list[:]".

Classes can use the same interfaces to control copying that they use
to control pickling. See the description of module "pickle" for
information on these methods. In fact, the "copy" module uses the
registered pickle functions from the "copyreg" module.

In order for a class to define its own copy implementation, it can
define special methods "__copy__()" and "__deepcopy__()". The former
is called to implement the shallow copy operation; no additional
arguments are passed. The latter is called to implement the deep copy
operation; it is passed one argument, the memo dictionary. If the
"__deepcopy__()" implementation needs to make a deep copy of a
component, it should call the "deepcopy()" function with the component
as first argument and the memo dictionary as second argument.

See also:

Module "pickle"
Discussion of the special methods used to support object state
retrieval and restoration.